This application is the national phase under 35 U.S.C. xc2xa7371 of prior PCT International Application No. PCT/IL98/00104 which has an International filing date of Mar. 3, 1998 which designated the United States of America, the entire contents of which are hereby incorporated by reference.
The present invention relates to wireless communication in general and to a method and a transmission system for improving performance, capacity and coverage in particular.
Wireless communication is known in the art. A conventional wireless transmitter produces an electromagnetic signal, which is transmitted over a medium. This medium is often non-perfect, being rural or urban, and induces reflections and other disturbances, which cause the signal to fade. This phenomena is called multi-path.
Cellular mobile communication attempts to provide mobility, multi-user capacity (many independent users access the system), coverage (service is offered over a large contiguous area) and grade and quality of service.
Cellular communications are generally limited by local codes to a range of frequencies. A widely used technique of cellular communications employs spatial isolation in order to be able to reuse the same frequencies beyond a given range called a guard zone. The communications of each user is maintained with a base station, whose antenna is elevated above the scenery in order to achieve a well defined and controlled coverage area. Sectorization is achieved by directive antennas that illuminate only one sector, thereby reducing interference, enhancing performance and reducing a pattern of frequency reuse.
The number of concurrent calls communicating with Each sector of cellular communications is limited by the frequency band assigned to the service, by the technology used and by the frequency reuse pattern. The number of calls per unit area, also called area capacity, can be increased by reducing the cell size. Small cells that are positioned below roof tops in urban areas are called microcells. These use lower and smaller antennas. The cell hardware is more compact, and in some cases has less circuits. Another technique for microcells involves the antenna and RF circuitry only, remote from the cell equipment and connected via RF, fiber or microwave link, to the cell. Such an arrangement is especially attractive for operators in possession of RF or fiber trunking, like CATV companies.
Electromagnetic radiation is polarized and allows for two orthogonal polarization states. It will be appreciated by those skilled in the art that an antenna can only transmit in a single polarization state.
The propagation of the signals through an inhomogeneous medium and through scattering may transfer part of the signal to the orthogonal polarization. This is the case for terrestrial communications, for example, and in particular in urban areas, where the signals encounter multipaths from objects on the way.
The transfer of polarization has been found to be typically xe2x88x9210 dB in rural areas, xe2x88x927.8 dB in urban areas and as high as xe2x88x924 dB indoors [See for example Jorn Toftgard and Patrick C. F. Eggers: Experimental Characterization of the Polarization State Dynamics of Personal Communication Radio Channels, Proc. IEEE VTC""93, pp.65-69].
The orthogonal polarization components have been found to have an independent fading pattern, with correlation lower than 0.6 and similar fading statistics.
Diversity techniques are used in wireless communications to mitigate the degradation due to signal fading. In space diversity, the antennas for space diversity are spaced apart enough for the fading of the waves arriving to each to be time-independent from those arriving at the other. The spacing required between the antennas is inversely proportional to the angle of intercept of the arriving waves. Accordingly, often the distance between such two antennas is chosen to be considerably large.
A repeater in the cellular system is a device that receives the transmission from the Base Station (the donor side) and retransmits it to the subscribers(the distribution side) with proper amplification. Simultaneously it receives the signals from the subscribers and retransmits it, with proper amplification, to the Base Station. Repeaters are used mainly for the following applications:
Providing RF coverage in areas where the signal received from the Base Station is too week (xe2x80x9cRadio Holesxe2x80x9d)
Extending the cell coverage, e.g. along highways
Extending the coverage into tunnels, buildings or other structures.
A repeater has to be transparentxe2x80x94the grade of service should not be degraded by the introduction of the repeater in the link. The repeater has to cover the frequency range allocated to the distribution area to be covered. Preferably it is the whole frequency range of the Base Station. The repeater has to have alarms, status reporting and controls, to be controlled from the Base Station, either via land lines or via transmissions. The lack of diversity in the repeaters hampers their performance.
It is an object of the present invention to provide a system for enhancing the coverage of wireless transmission, which overcomes the disadvantages of the prior art.
It is another object of the present invention to provide a novel method for controlling the polarization of a transmitted signal, thereby overcoming the disadvantages of the prior art.
In accordance with a preferred embodiment of the present invention, there is thus provided a modular cellular wireless communication base station, which includes a plurality of active radiator modules located at a desired antenna location, wherein each module includes at least one antenna for transmitting and receiving, a transmitter including a power amplifier, and a receiver.
The base station further includes a beam forming network controlling relative amplitudes and phases of each of the modules, an RF front end transmitting over a low power link with the active radiator modules via the beam forming network and receiving over a lower power link via a low noise amplifier.
The base station further includes a delay diversity module that provides a transmission CDMA delay diversity.
According to one aspect of the invention, the delay diversity module includes a SAW delay line and an amplifier that compensates for a delay line insertion loss.
According to another aspect of the invention, the delay diversity module is connected at a transmission beam forming network input.
According to a further aspect of the invention, the delay diversity module is connected at an active antenna transmission input.
A polarization diversity and matching system for cellular radio, including:
a dual polarized antenna pair at a base station, each antenna including an appropriate receive channel; and
a signal combining and control circuitry that adds polarization diversity to a base station receiver.
Preferably, the circuitry is characterized by two time constants, wherein a fast circuit adapts to fading signals on a received reverse link and changes weights of two antennas, and a slow circuit follows physical movements of a mobile station and averages fading of a received signal.
The slow circuit is driven by information from received signals and matches transmitted signal polarization to that of an incoming signal.
The slow circuit can also detect an average polarization vector direction from received signals and produces an output matched signal from the received signals according to the average polarization vector direction.
Signal portions from two receiving antennas are weighed by weights controlled by the signal combining and control circuitry, the weights being fed into a transform circuit that transforms the weights according to polarizations of the transmitting antennas and differences in gain.
The system can further include a low-pass filter that averages fast control variations and responds only to slow variations resulting from a physical attitude change of the station.
Preferably, a transmit chain is split into two branches, and weights are applied to each branch for each receive channel.
Alternatively, a transmit chain is split into two branches that operate at a fixed power. Accordingly, the phase of one of the branches is switched between 0xc2x0, +90xc2x0, +180xc2x0, xe2x88x9290xc2x0, and xe2x88x92180xc2x0.
In accordance with a further preferred embodiment of the invention, there is thus provided a method for increasing transmission gain to a mobile station of a mobile communications system. The method includes the steps of:
substantially simultaneously transmitting from two transmit antennas so as to form a radiation pattern that is characterized by a plurality of radiation lobes. Each lobe is characterized by a width inversely proportional to a distance between the antennas. The amplitude of the lobes is bound by the radiation pattern of the antennas,
determining a transmission direction to a mobile station, and
aiming the pattern so as to produce a maximum in the transmission direction, thereby increasing transmission gain to the mobile station, and reducing scattering into the mobile station from foreign objects.
The step of determining a transmission direction can include amplifying and filtering a signal from each antenna, splitting the signals, and changing a phase of the signals relative to one another so as to determine the direction.
The step of determining a transmission direction can include extracting direction information from a receive diversity control for a given antenna channel and correcting for a difference in frequency.
In accordance with another aspect of the present invention there is thus provided an apparatus for increasing transmission gain to a mobile station of a mobile communications system. The apparatus includes two transmit antennas positioned together with a pair of diversity receive antennas, and an aiming apparatus.
The two transmit antennas transmit substantially simultaneously so as to form a radiation pattern that is characterized by a plurality of radiation lobes. Each lobe is characterized by a width inversely proportional to a distance between the transmit antennas. The amplitude of the lobes is bound by the radiation pattern of the transmit antennas.
The aiming apparatus aims the pattern so as to produce a maximum in a transmission direction to a mobile station, thereby increasing transmission gain to the mobile station, and reducing scattering into the mobile station from foreign objects.
The transmit antennas and the receive antennas can include a plurality of active radiator module arrays.
Furthermore, the transmit antennas are preferably spaced by a distance as required to avoid correlation between fading of signals from remote mobile stations within a coverage area.
In accordance with a further aspect of the present invention there is thus provided a system for improving coverage of a wireless transceiver. The system includes a means for transmitting an outgoing is signal in two orthogonal polarized states; and means for controlling the polarization of at least one of the states, thereby polarizing the outgoing signal at any desired direction.
The transmitting means can include a plurality of antennas, wherein at least one of the antennas transmits a portion of the outgoing signal in a predetermined polarization vector, and at least another one of the antennas transmits a portion of the outgoing signal in a polarization vector which is orthogonal to the predetermined polarization vector.
The controlling means constantly change the polarization direction of the outgoing signal.
In accordance with another aspect of the present invention there is thus provided a method for improving coverage of a wireless transmitter including the steps of:
receiving an outgoing signal,
constantly changing the direction of the polarization vector of the outgoing signal, according to a polarization change pattern, and
transmitting the polarized outgoing signal.
The method can further include the step of determining the polarization change pattern.
The polarization change pattern can be determined from the characteristics of the wireless transmitter, or from the characteristics of the area covered by the wireless transmitter, or from both.
The method can further include the step of determining the characteristics of the area.
Accordingly, the polarization change pattern is selected from the list consisting of a linear polarization change pattern, a cyclic polarization change pattern, a non-linear polarization change pattern, a random polarization change pattern, and the like.
In accordance with another aspect of the present invention there is thus provided a repeating device including a donor antenna for linking to a transmitting antenna of a base station, two orthogonally polarized donor antennas, for linking to two orthogonally polarized receive antennas of the base station, a subscriber-side antenna, for linking to at least one mobile transceiver and two orthogonally polarized subscriber-side antennas, for linking to the at least one mobile transceiver.
The device further includes an amplifier, connected between the donor antenna and the subscriber-side antenna, for amplifying a signal received by the donor antenna prior to transmitting the signal via the subscriber-side antenna, another amplifier, connected between one of the orthogonally polarized donor antenna and one of the orthogonally polarized subscriber-side antenna, for amplifying a signal received by the one orthogonally polarized subscriber-side antenna prior to transmitting the signal via the one orthogonally polarized donor antenna and a further amplifier, connected between the other of the orthogonally polarized donor antenna and the other of the orthogonally polarized subscriber-side antenna, for amplifying a signal received by the other orthogonally polarized subscriber-side antenna prior to transmitting the signal via the other orthogonally polarized donor antenna.
In accordance with a further aspect of the present invention there is thus provided a repeating device including two orthogonally polarized donor transmit antennas, for linking to two orthogonally polarized receive antennas of a base station, two orthogonally polarized donor receive antennas, for linking to two orthogonally polarized transmit antennas of the base station, two orthogonally polarized subscriber-side transmit antennas, for linking to at least one mobile transceiver and two orthogonally polarized subscriber-side receive antennas, for linking to the at least one mobile transceiver.
The device further includes an amplifier, connected between one of the orthogonally polarized donor transmit antenna and one of the orthogonally polarized subscriber-side receive antennas, for amplifying a signal received by the one orthogonally polarized subscriber-side receive antenna prior to transmitting the signal via the orthogonally polarized donor transmit antenna.
The device also includes another amplifier, connected between the other of the orthogonally polarized donor transmit antennas and the other of the orthogonally polarized subscriber-side receive antennas, for amplifying a signal received by the other orthogonally polarized subscriber-side receive antenna prior to transmitting the signal via the other orthogonally polarized donor transmit antenna.
The device further includes an amplifier, connected between one of the orthogonally polarized donor receive antenna and one of the orthogonally polarized subscriber-side transmit antennas, for amplifying a signal received by the one orthogonally polarized donor receive antenna prior to transmitting the signal via the orthogonally polarized subscriber-side transmit antenna.
The device also includes an amplifier, connected between the other of the orthogonally polarized donor receive antenna and the other of the orthogonally polarized subscriber-side transmit antennas, for amplifying a signal received by the other orthogonally polarized donor receive antenna prior to transmitting the signal via the other orthogonally polarized subscriber-side transmit antenna.
In accordance with a further aspect of the present invention, there is thus provided a method for repeating a randomly polarized signal, including the steps of:
receiving the signal at first and second states, thereby providing a first portion of the signal, received at the first polarization state and a second portion of the signal, received at the second polarization state, and
transmitting the first portion according to the first polarization state and the second portion according to the second polarization state.
This method can further include the step of amplifying the first and second portions, before the step of transmitting.
Preferably, the first polarization state is orthogonal to the second polarization state.
In accordance with another aspect of the present invention, there is provided a repeating device including a donor side transceiver section, a subscriber side transceiver section and amplification means connected therebetween.
The donor side includes a plurality of donor side transceiver elements, transmitting outgoing signals in a first non-correlated manner.
The subscriber side includes a plurality of subscriber side transceiver elements, receiving incoming signals in a second non-correlated manner.
According to one aspect of the invention at least one of the first non-correlated manner and the second non-correlated manner incorporates space diversity. According to one aspect of the invention at least one of the first non-correlated manner and the second non-correlated manner incorporates polarization diversity. Thus, any combination of such manner is applicable for the present invention.